2 large bottles (and the option to refill them along the way) has gotten me through every ride I have done, including summers in Oman (where it would sometimes be 50C when i started cycling). I hate Camelbacks - they arent comfy, my back gets sweaty and they just look a bit daft, to be honest. That said, if there is no other way and you need the water, carry the Camelback. Practicality always wins.
Are triathlete-style saddleback bottle holders and option? If you arent riding in a paceline, you could carry your bottles there instead.